In recent years, there has been an increasing demand for thin, lightweight, and fast response display devices. Correspondingly, research and development for organic EL (Electro Luminescence) displays and FEDs (Field Emission Displays) have been actively conducted.
Organic EL elements included in an organic EL display emit light at higher luminance with a higher voltage applied thereto and a larger amount of current flowing therethrough. However, the relationship between the luminance and voltage of the organic EL elements easily fluctuates by the influence of drive time, ambient temperature, etc. Due to this, when a voltage control type drive scheme is applied to the organic EL display, it is very difficult to suppress, variations in the luminance of the organic EL elements. In contrast to this, the luminance of the organic EL elements is substantially proportional to current, and this proportional relationship is less susceptible to external factors such as ambient temperature. Therefore, it is desirable to apply a current control type drive scheme to the organic EL display.
Meanwhile, pixel circuits and drive circuits of a display device are formed using TFTs (Thin Film Transistors) composed of amorphous silicon, low-temperature polycrystal silicon, CG (Continuous Grain) silicon, etc. However, variations are likely to occur in TFT characteristics (e.g., threshold voltage and mobility). Hence, a circuit that compensates for variations in TFT characteristics is provided in a pixel circuit of an organic EL display. By the action of this circuit, variations in the luminance of an organic EL element are suppressed.
Schemes to compensate for variations in TFT characteristics in the current control type drive scheme are broadly classified into a current program scheme that controls the amount of current flowing through a driving TFT by a current signal; and a voltage program scheme that controls such an amount of current by a voltage signal. By using the current program scheme variations in threshold voltage and mobility can be compensated for, and by using the voltage program scheme only variations in threshold voltage can be compensated for.
The current program scheme, however, has the following problems. First, since a very small amount of current is handled, it is difficult to design pixel circuits and drive circuits. Second, since the influence of parasitic capacitance is likely to be received while a current signal is set, it is difficult to achieve an increase in area. On the other hand, in the voltage program scheme, the influence of parasitic capacitance, etc., is very small and a circuit design is relatively easy. In addition, the influence of variations in mobility exerted on the amount of current is smaller than the influence of variations in threshold voltage exerted on the amount of current, and the variations in mobility can be suppressed to a certain extent in a TFT fabrication process. Therefore, even with a display device to which the voltage program scheme is applied, sufficient display quality can be obtained.
For an organic EL display to which the current control type drive scheme is applied, various configurations have been conventionally known. For example, Patent Document 1 describes that a pixel circuit 100 shown in FIG. 2 (details will be described later) is driven according to a timing chart shown in FIG. 13. In a drive method shown in FIG. 13, before time t1, the potentials of a scanning line Gi and a control wiring line Wi are controlled to a high level, the potential of a control wiring line Ri to a low level, and the potential of a data line Sj to a reference potential Vpc. When at time t1 the potential of the scanning line Gi is changed to a low level, a switching TFT 111 changes to a conducting state. Then, when at time t2 the potential of the control wiring line Wi is changed to a low level, a switching TFT 112 changes to a conducting state. By this, the gate and drain terminals of a driving TFT 110 are short-circuited and reach the same potential.
Then, when at time t3 the potential of the control wiring line Ri is changed to a high level, a switching TFT 113 changes to a non-conducting state. At this time, a current flows into the gate terminal of the driving TFT 110 from a power supply wiring line Vp through the driving TFT 110 and the switching TFT 112, and thus the gate terminal potential of the driving TFT 110 rises while the driving TFT 110 is in a conducting state. Since the driving TFT 110 changes to a non-conducting state when the gate-source voltage thereof reaches a threshold voltage Vth (negative value), the gate terminal potential of the driving TFT 110 rises to (VDD+Vth).
Then, when at time t4 the potential of the control wiring line Wi is changed to a high level, the switching TFT 112 changes to a non-conducting state. At this time, a potential difference (VDD+Vth−Vpc) between the gate terminal of the driving TFT 110 and the data line Sj is held in a capacitor 121.
Then, when at time t5 the potential of the data line Sj is changed from the reference potential Vpc to a data potential Vdata, the gate terminal potential of the driving TFT 110 changes by the same amount (Vdata−Vpc) and reaches (VDD+Vth+Vdata−Vpc). Then, when at time t6 the potential of the scanning line Gi is changed to a high level, the switching TFT 111 changes to a non-conducting state. At this time, a gate-source voltage (Vth+Vdata−Vpc) of the driving TFT 110 is held in a capacitor 122.
Then, at time t7, the potential of the data line Sj changes from the data potential Vdata to the reference potential Vpc. Then, when at time t8 the potential of the control wiring line Ri is changed to a low level, the switching TFT 113 changes to a conducting state. By this, a current flows to an organic EL element 130 from the power supply wiring line Vp through the driving TFT 110 and the switching TFT 113. The amount of current flowing through the driving TFT 110 increases and decreases according to the gate terminal potential thereof (VDD+Vth+Vdata−Vpc). Even if the threshold voltage Vth is different, if the potential difference (Vdata−Vpc) is the same, then the amount of current is the same. Therefore, regardless of the value of the threshold voltage Vth, a current of an amount according to the data potential Vdata flows through the organic EL element 130, and thus the organic EL element 130 emits light at a luminance according to the data potential Vdata.
Accordingly, by driving the pixel circuit 100 shown in FIG. 2 according to the timing chart shown in FIG. 13, regardless of the threshold voltage Vth of the driving TFT 110, a current of a desired amount is allowed to flow through the organic EL element 130, and thus the organic EL element 130 is allowed to emit light at a desired luminance.
Patent Document 2 describes that a pixel circuit 900 shown in FIG. 14 is driven according to a timing chart shown in FIG. 15 (note that, for easy contrast with the present invention, the names of signal lines are changed). In a drive method shown in FIG. 15, before time t1, the potentials of scanning lines G1l and G2i are controlled to a high level, and the potential of a control wiring line Ei to a low level. When at time t1 the potential of the control wiring line Ei is changed to a high level, switching TFTs 913 and 914 change to a non-conducting state. Then, when at time t2 the potentials of the scanning lines G1l and G2i are changed to a low level, switching TFTs 911, 912, and 915 change to a conducting state. By this, the gate and drain terminals of a driving TFT 910 are short-circuited and reach the same potential, and a gate terminal potential Vg of the driving TFT 910 becomes equal to a potential Vpc of a power supply wiring line Vint. In addition, a potential Vdata of a data line Sj is applied to a connection point between the switching TFT 911 and a capacitor 921 (hereinafter, referred to as a connection point B).
Then, when at time t3 the potential of the scanning line G2i is changed to a high level, the switching TFT 915 changes to a non-conducting state. At this time, a current flows into the gate terminal of the driving TFT 910 from a power supply wiring line Vp through the driving TFT 910 and the switching TFT 912, and thus the gate terminal potential Vg of the driving TFT 910 rises while the driving TFT 910 is in a conducting state. Since the driving TFT 910 changes to a non-conducting state when the gate-source voltage thereof reaches a threshold voltage Vth (negative value), the gate terminal potential Vg of the driving TFT 910 rises to (VDD+Vth).
Then, when at time t4 the potential of the scanning line G1i is changed to a high level and the potential of the control wiring line Ei is changed to a low level, the switching TFTs 911 and 912 change to a non-conducting state, and the switching TFTs 913 and 914 change to a conducting state. At this time, the potential at the connection point B changes from Vdata to Vpc, and the gate terminal potential Vg of the driving TFT 910 changes by the same amount as the potential at the connection point B and reaches (VDD+Vth+Vpc−Vdata). The capacitor 921 holds a potential difference (VDD+Vth−Vdata) between the gate terminal of the driving TFT 910 and the power supply wiring line Vint.
After time t4, a current flows to an organic EL element 930 from the power supply wiring line Vp through the driving TFT 910 and the switching TFT 913. The amount of current flowing through the driving TFT 910 increases and decreases according to the gate terminal potential thereof (VDD+Vth+Vpc−Vdata). Even if the threshold voltage Vth is different, if the potential difference (Vpc−Vdata) is the same, then the amount of current is the same. Therefore, regardless of the value of the threshold voltage Vth, a current of an amount according to the data potential Vdata flows through the organic EL element 930, and thus the organic EL element 930 emits light at a luminance according to the data potential Vdata.
Accordingly, by driving the pixel circuit 900 shown in FIG. 14 according to the timing chart shown in FIG. 15, regardless of the threshold voltage Vth of the driving TFT 910, a current of a desired amount is allowed to flow through the organic EL element 930, and thus the organic EL element 930 is allowed to emit light at a desired luminance.
Note that examples of the organic EL display to which the current control type drive scheme is applied are also described in Patent Document 3 and another application (Japanese Patent Application No. 2008-131568, filed on May 20, 2008) having a common applicant and a common inventor with the present application.